a) Their gun failed them. Not the caliber but the gun itself. A jammed rifle can lead to many bad results, particularly when your rifle sights were pointed towards a charging elephant a mere moments before.
b) Their nerve failed them. After all having a charging Cape buffalo erupt from the thicket 9 feet to your left tends to give many the creeps. Now imagine being on foot, in an African thicket (which can ahve white thorns the size of fingers), and you know there is a wounded buffalo in there somewhere. And then you detect movement. Many people will have their nerves fail them, especially once you consider the sheer size of some of those critters.
c) Shot placement: Going back to the Cape Buffalo example. Let's say you shoot, and your slug hits the buffalo's boss (where its great horns converge on the top of the head). All you did is give it a mild headache, and a raging bloodthirst. Chances are you only have one more shot before it gets to you. This is why most of the great hunters used to use double rifles ....powerful, with 2 shots that have virtually no chance of jamming. And 2 shots are more than enough, because if you miss with both of them you would be dead either way. Shot placement counts, and this is even more paramount when it comes to elephant. Unless you are basically murdering the beast (eg using kalashnikovs) instead of hunting, shot placement will really count. Many people with good guns and powerful rounds died due to being an inch or two off. The bush does not forgive such errors.
d) Mr Murphy and his ludicrous laws. There are stories of people shooting lions right through the heart and it still manages to get to them and kill them. Or someone hunting leopard and wounding it, just as the sun is setting. Tracking down a wounded leopard at night (or low light) is directly tantamount to suicide. A leopard may be the size of a cougar, but leopards are regarded as the most efficient man-eaters (and they actually have a larger tally of people than tigers and lions), while all a cougar does is pick off some old lady or young'un every 6 months or so. Another example of Murphy scr#wing up one's day is if one forgets to put off the safety. Sound stupid, but in the rush of things ...particularly when you have a 500 pound lion charging you at 50 miles an hour ....such 'simple stuff' have led to the death of professionals. OR, the inverse, having someone behind you with his safety off and a nervous twitch that has a direct line to his trigger finger ....and that finger twitches. Again, better men have been killed by that.
Anyways it all boils down to skill as well as a bit of luck, and not just in terms of weapon choice but also hunting strategy. For example any fool with a scoped rifle can bring down the most blood-thirty lioness from a distance, however the game changes if the hunt is taking place in thick woodland, an it totally flips if the hunt is in dense shrub. Killing a lion can be far easier than shooting duck (if one is in a plain), or it can be more difficult than finding a virgin nun at an Amsterdam brothel (if the same beast is in scrub). Oh, and if you shoot at a lion and miss then you can be sure it will never make that mistake again. That is why man-eating lions are so darn hard to kill. They learn very quick never to make mistakes, and thus can take forever to put down since they show an almost demonic intellect.
Anyways, the proposed idea is rather silly. Introducing exotic fauna (or flora even) to new locales almost always leads to problems. Particularly when one is introducing apex predators (eg lions and leopards), niche predators (eg cheetah), as well as large animals (eg elephant, which are part of the big 5 for a reason). What will happen is either the project will be a disaster in terms of the animals dying due to an ill fit with the environs, or a disaster in terms of people getting killed (either by 'accident,' studipidity, or just plain bad luck).
Anyways, imagine a nice lil' picnic and a big buffalo like this one appearing. It would be easy to put it down from a distance with a scoped rifle, but at close range you'd better hope your chips are right with St Pete.
